472 Dr. G. Owen and Mr. H. Pealing on Condensation 



iodine decreases much more rapidly when the apparatus is 

 kept unshielded in bright diffused daylight than when kept 

 in the dark. If the apparatus be taken down when in this 

 non-sensitive state and thoroughly rinsed out with distilled 

 wafer, the large effects already described once more make 

 their appearance when iodine is admitted. 



The following possible causes of the disappearance of the 

 effect naturally suggested themselves and were investigated 

 in turn : — 



(1) That some change took place in the properties of the 

 iodine itself, say through its becoming damp. Intro- 

 ducing fresh iodine, however, failed to bring back the 

 original clouds. 



(2) Thinking that possibly the nuclei might be due to an 

 action of the iodine vapour on some impurity brought 

 into the apparatus when it was filled with air, and that 

 the disappearance of the effect in the course of a few 

 days was due to this impurity being all used up, we 

 tried admitting a fresh supply of air. On two or three 

 occasions fresh air did partly bring back the clouds, but 

 generally this was not the case, both air from boiling liquid 

 air and dusty laboratory air being equally in effective. 



(3) Again, it was thought possible that the original clouds 



were due to an action of the iodine on vapours evolved 

 by the vaseline lubricating the taps. But we found 

 that introducing fresh vaseline into the apparatus pro- 

 duced no change. Various other possible sources of 

 impurity, such as tap-grease, indiarubber, red wax, 

 cotton-wool, were placed in the apparatus. In every 

 case there was no increased effect. 



(4) Again, we thought that the film of iodine which natura^y 



forms in time on the sides of the cloud-chamber might 

 possibly cut off' the effective part of the light entering 

 the bulb. But driving off this film by gently heating the 

 glass failed to bring back the original clouds. 



(5) Again, it is well known that the value of the expansion 



required to catch nuclei of a given size depends upon 

 the nature and condition of the liquid in the cloud- 

 chamber. Now, after the iodine has been in the 

 apparatus for two or three days, the water in the cloud- 

 chamber develops a bright yellow colour owing to the 

 solution in it of some of the iodine vapour. That the 

 disappearance of the clouds is not due to this change in 

 the water was shown in two ways : — 

 (a) The coloured water in the cloud-chamber was drained 

 eff through the tap E and fresh distilled water admitted 

 through D, This process, however, had no effect. 



